With recent utilization of large screen inline color CRT's for both CAD/CAM and entertainment applications, a reduced electron beam spot size over the entire screen is required for the high resolution requirements of such applications. The color display system includes the inline color CRT and a self-converging yoke, for providing magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen of the tube. Because of fringe fields, the self-converging yoke introduces into the tube strong astigmatism and deflection defocusing caused primarily by vertical overfocusing and, secondarily, by horizontal underfocusing of the beams during deflection.
To compensate, it has been the practice to introduce an astigmatism into the beam-forming region of the electron gun to produce a defocusing of the vertical rays and an enhanced focusing of the horizontal rays. Such astigmatic beam-forming regions have been constructed by means of G1 control grids or G2 screen grids having slot-shaped apertures. These slot-shaped apertures produce non-axially-symmetric fields with quadrupolar components which act differently upon rays in the vertical and horizontal planes. Such slot-shaped apertures are shown in U.S. Pat. No. 4,234,814, issued to Chen et al. on Nov. 18, 1980. These constructions are static; the quadrupole field produces compensatory astigmatism even when the beams are undeflected and experiencing no yoke astigmatism.
To provide improved dynamic correction, U.S. Pat. No. 4,319,163, issued to Chen on March 9, 1982, introduces an extra upstream screen grid, G2a, with horizontally slotted apertures, and with a variable or modulated voltage applied to it. The downstream screen grid, G2b, has round apertures and is at a fixed voltage. The variable voltage on G2a varies the strength of the quadrupole field, so that the astigmatism produced is proportional to the scanned off-axis position.
Although effective, use of astigmatic beam-forming regions has several disadvantages. First, beam-forming regions have a high sensitivity to construction tolerances because of the small dimensions involved. Second, the effective length or thickness of the G2 grid must be changed from the optimum value it has in the absence of slotted apertures. Third, beam current may vary when a variable voltage is applied to a beam-forming region grid. Fourth, the effectiveness of the quadrupole field varies with the position of the beam cross-over and, thus, with beam current.
U.S. Pat. No. 4,731,563 issued to Bloom et al. on March 15, 1988 discloses an astigmatism correction for an electron gun which is not subject to the enumerated disadvantages. The gun includes beam-forming region electrodes, main focusing lens electrodes, and two interdigitated electrodes for forming a multipole lens between the beam-forming region and the main focusing lens in each of the electron beam paths. Each multipole lens is oriented to provide a correction to an associated electron beam to at least partially compensate for the effect of the astigmatic magnetic deflection field on that beam. A first multipole lens electrode is located between the beam-forming region electrodes and the main focusing lens electrodes. A second multipole electrode is connected to a main focusing lens electrode and located between the first multipole lens electrode and the main focusing lens, adjacent to the first multipole lens electrode. Means are included for applying a fixed focus voltage to the second multipole lens electrode and a dynamic voltage signal, related to the deflection of the electron beams to the first multipole lens electrode. Each multipole lens is located sufficiently close to the main focusing lens to cause the strength of the main focusing lens to vary as a function of voltage variation of the dynamic voltage signal. The dynamic voltage signal modulates the first multipole lens electrode at the horizontal scan rate to correct the distortion of the electron beams at the 3:00 and 9:00 o'clock (hereinafter the 3D and 9D) screen locations with a single waveform. However, because of the penetration of the fringe fields into the electron gun, the beams are caused to pass off-axis through a stronger part of the main focusing lens. The off-axis paths of the beams and the vertical overfocusing action caused by the vertical deflection windings of the self-converging yoke require a higher vertical focus voltage at the top of the screen than at the center of the screen, and dynamic correction of this focus voltage difference must be achieved at the vertical scan rate. This can be achieved using the interdigital structure within the main focusing lens; however, because of the low vertical rate frequency (60 Hz), it is difficult to economically capacitively couple the required waveform into the focus supply without degrading the tracking characteristics of the focus supply with respect to the anode supply
U.S. Pat. No. 4,764,704 issued to New et al. on Aug. 16, 1988 utilizes the dynamically modulated multipole lens of U.S. Pat. No. 4,731,563 in combination with an additional lens located between the beam-forming region of the electron gun and the multipole lens. The additional lens provides a static correction and refraction of the electron beams emerging off axis from the lens of the beam-forming region and asymmetrically focuses the beams to provide asymmetrically-shaped beams to the main focus lens. A drawback of the additional lens is that the rectangularly-shaped apertures that are utilized to provide static correction to the beams are difficult to align accurately on the cylindrical mount pins used during electron gun fabrication.
T. Katsuma et al. in an article entitled, DYNAMIC ASTIGMATISM CONTROL QUADRA POTENTIAL FOCUS GUN FOR 21-IN. FLAT SQUARE COLOR DISPLAY TUBE, SID DIGEST, 136 (1988), describe a Quadra Potential Focus gun having six electrodes with the fourth (G4) electrode comprising three discrete element G41, G42, and G43. A dynamic voltage with a parabolic wave form is applied to the G2 electrode and to the G41 and G43 elements of the G4 electrode. The G42 element has vertically oriented oval apertures which in conjunction with the horizontal blades located above and below the round apertures of the G41 and G43 elements, facing the G42 element, form a quadrupole lens that provides adequate compensation for astigmatism and deflection defocusing. A drawback of the described gun is that the number of parts has been increased, adding to the cost of the gun, and the oval apertures in the G42 element pose the same difficulty in alignment as the rectangular apertures of U.S. Pat. No. 4,764,704.
A variation of the gun of Katsuma et al. is described in an article entitled, QUADRUPOLE LENS FOR DYNAMIC FOCUS AND ASTIGMATISM CONTROL IN AN ELLIPTICAL APERTURE LENS GUN, S. Shirai et al., SID DIGEST, 162 (1987). The quadrupole lens of the Shirai et al. gun is also formed by a three element G4 electrode. The quadrupole lens is formed by rotationally asymmetrical through-holes in the G42 element and horizontal slots around the circular apertures of the G41 and G43 elements of the G4 electrode. A dynamic voltage is applied to the G41 and G43 elements. A disclosed drawback of the gun is that the astigmatism correction ability of the quadrupole lens is limited by the aberration of the main lens.